GUELPH FAUNA IN THE STATE OF NEW YORK 51 



localities, so that we may conclude that it reached its acme there and sur* 

 vived only sparsely in the Guelph, where it has been found occasionally at 

 Hespeler and Elora, though in greater number in the lower Shelby dolomite. 

 The series of orthoceracones from the lower Shelby bed contains a 

 fragment, consisting of seven chambers, which appears to be a portion of 

 the apical part of the conch of O. crebescens, which has hitherto not 

 been observed. It agrees with that species and differs from the other asso- 

 ciate orthoceratites by its slender form, very deep camerae, their depth 

 being nearly half the width of the conch, and the very large siphuncle, 

 which occupies nearly half the interior space of the conch. This siphuncle, 

 however, is but very slightly expanded, and the cast of the siphuncular 

 cavity is completely cylindric. As the siphuncle of other species is known 

 to become nummuloidal only in late growth stages, the difference between 

 the siphuncles of this small conch and those of the large conchs of O. 

 crebescens is not thought to militate against a reference of the same 

 to that species. 



DAWSONOOERAS Hyatt. 1 883 



Dawsonoceras annulatum Sowerby, var. americanum Foord 



Plate 10, fig. 19-21 ; plate n, fig. 1 



Orthoceras annulatum Sowerby var. americanum Foord, Cat. Fossil 

 Cephal. British Museum. 1888. p. 56, 57 



The wide variation in the expression of the ornament among specimens 

 which have been referred to O. annulatum Sowerby and have likewise 

 been described under a variety of other names, now renders exceedingly 

 difficult the accurate determination of specimens of this type, specially 

 when the species are represented by only fragmentary parts. It is evident 

 from the study of the long list of annulated orthoceracones of the late 

 Siluric, and has been specifically pointed out 1 by one of the writers in the 

 case of certain lower Siluric shells of similar type, that the ontogenic 

 progress of the ornament in the annulate and longitudinally striate shells is 

 quite uniform, and it may be summarized thus : The annuli are a quite 



'Minnesota Paleontology, v. 3, pt 2, p. 787. 



